Red Diamond (Luigi's Mansion)
From the Super Mario Wiki
Red Diamonds are treasures in Luigi's Mansion. Surprisingly, unlike the Silver and Gold Diamonds, they are extremely low in value, with each Red Diamond worth only 5,000G, the same amount as a coin. There are only two, and one of them is King Boo's crown, automatically given to Luigi when he sucks up King Boo after his battle on the roof. The other is found by watering a plant on the Balcony.